JPH01108187A - Oxidation resistance high temperature resistance cyclic coating on silicon substrate and manufacture - Google Patents

Abstract

PURPOSE: To obtain coated articles which are useful for gas turbine rotors, etc., and have excellent long-term adhesion property, oxidation resistance and high-temp. strength by forming a continuous high-density intermediate layer consisting of AlN, etc., on a high-density molded substrate body and a continuous high-density first outer layer consisting of Al₂O₃, etc., thereon, respectively as specific compsn. graded layers.

CONSTITUTION: The continuous high-density intermediate layer consisting of an aluminum (oxy)nitride having a thickness of 0.5 to 20 μm is formed on the high-density molded substrate body consisting of an Si₃N₄ base or SiC base alone or composite material by passing a gaseous mixture composed of ≥1 kinds of halide vapors and other reactant gases and, if necessary, a carrier gas, at 900 to 1,500°C under a pressure of 1 Torr to ambient pressure and effecting the chemical bond while controlling the partial pressure of the vapors and the gases in such a manner that the materials adhere as such compsn. graded layers where the distinctly recognizable compsn. boundaries do not exist between the substrate and the layers and between the layers from the materials on the lower side to be adhered up to the adhered material layers thereon. Next, the tightly adhered continuous high-density first outer layer having a thickness of 0.5 to 900 μ and contg. Al₂O₃ or ZrO₂ is similarly formed thereon, by which the coated articles are obtd.

COPYRIGHT: (C)1989,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an article comprising a compositionally graded coating deposited on a hard silicon-based ceramic substrate;
In particular, it relates to coated articles having oxidation resistance and high temperature strength. The coating consists of at least two compositionally graded layers on such a substrate.6 The present invention also relates to a method of manufacturing such an article.

Hard silicon-based ceramic materials are well known and widely used in metal cutting and drilling tools, metal extrusion gougs, wear-resistant mechanical parts, and other applications. Such silicon-based materials include silicon nitride and silicon carbide materials, both alone and as composites. The service properties of such materials, such as wear, heat and chemical resistance, can be improved by applying one or more thin coatings of, for example, metal carbides, metal nitrides or other types of ceramics. Cutting tools made of coated silicon nitride-based materials are disclosed in U.S. Pat.
, No. 431.431 (Metal Carbide Coating), No. 4
．． No. 406.667 and 4.406.668 (Metal Nitride Coating) 9,003 and 409,004 (Metal Carbonitride Coating), No. 4,421.525
and 4.440.547 (alumina coating) and 4.441.894 and 4,449,989 (alumina outer coating with metal carbide, nitride or carbonitride lower coating). Much research has been done into improving the performance of these coated substrates, for example in machining applications, by micronization of the substrate composition and by the application of various combinations of overlapping layers of coating material.

These high-tech ceramic materials offer the potential to exhibit good performance properties for high temperature applications due to their mechanical properties and fracture toughness. However, as components for modern gas turbine engines, they do not exhibit satisfactory resistance to contact stresses, including sliding contact stresses, nor satisfactory resistance to high temperature oxidation under contact stress conditions. Ta.

Oxide coatings are known to provide oxidation resistance to various materials at high temperatures. However, contact stress damage at the ceramic-ceramic interface is still the predominant failure mode for ceramic heat engine components. Researchers have found that alumina or zirconia type oxide coatings approximately 50-100 microns thick provide improved protection against such surface damage and strength loss under sliding contact. However, such coatings did not exhibit sufficient adhesion under long-term use in heat engine applications. Thermal barrier coatings applied by physical vapor deposition have been developed for diesel engine components. The coating has a pound coat approximately 100-200 microns thick with a metal (Fe%CO or N1)-chromium-aluminum-yttrium composition. A protective yttoria stabilized zirconia coating approximately 800 microns thick is coated over the bond coat. However, these coatings on most parts are inherently porous, unless by electron beam evaporation.

Components used in heat engines encounter harsh thermal cycling conditions. Here, parts are repeatedly subjected to cycling between ambient temperatures and temperatures that can reach temperatures as high as 1400° C. over extended periods of use. Differences in the coefficient of thermal expansion between the materials between the substrate and the coating, and between the various coating layer materials in multilayer coatings, induce high tensile and compressive stresses during the coating. These stresses also depend on the thickness of the coating. There is a risk that excessive stress can lead to thermal shock and cracking and peeling of the coating under such severe usage conditions.

Additionally, the brittleness and hardness of silicon-based materials and oxide coatings can create severe localized surface stresses. The increase in stress is primarily due to the hardness of the material preventing stress redistribution. If the local stress exceeds the nominal strength of the material, the surface of the part will be damaged and the strength and oxidation resistance of the part will also be reduced.

Therefore, it exhibits long-term adhesion to hard silicon-based ceramic substrates, exhibits resistance to sliding contact in oxidizing environments and/or under high temperature thermal cycling conditions, and exhibits the problems described above. There is an urgent need to develop ceramic coatings that do not.

It is an object of the present invention to provide a novel coated ceramic article that satisfies these needs.

Light jΔ eyes 1! The coated article according to the invention is based on a fully dense molded substrate body of silicon nitride-based or silicon carbide-based single or composite material.

A coherent, continuous, dense intermediate layer of aluminum nitride or aluminum oxynitride material, about 0.5 to 20 microns thick, is deposited and chemically bonded onto the substrate body.

Approximately 0 containing aluminum or zirconium oxides
．． A coherent, continuous, dense first outer layer, 5 to 900 microns thick, is deposited and chemically bonded onto the first intermediate layer.

Each coating layer is distinct from the underlying material to which it is deposited to the material of that layer and to the layer of material deposited thereon, if any, between the substrate and the layer and between the layers. It is a compositionally graded layer that varies in composition such that there are no vine compositional interfaces.

The present invention also provides a method of depositing a high temperature stress and oxidation resistant coating on a silicon nitride-based or silicon carbide-based substrate body, which coats the substrate with approximately 900 to 15
00C and at pressures from about 1 Torr to about ambient pressure. The gas mixture is flowed over the substrate at a sufficient partial pressure and flow rate for a sufficient time to deposit a continuous, dense, adherent coating on the substrate. The halide vapor and other suitable reactant gases are present during coating deposition, where the coating has a compositional gradient comprising at least two layers, each of which increases the concentration of said layer from the underlying material to which it is deposited. A compositional gradient that varies in composition such that there are no discernible compositional interfaces between the substrate and the layers and between the layers, up to the material and any layers of material deposited thereon. It is graded in such a way that it is deposited as a layer. The vapors and gases and their partial pressures are varied according to a predetermined time schedule and the halide vapor and other suitable reactant gases are selected accordingly to obtain the article as described above.

Articles according to the invention have a compositional gradient of two or more layers on a hard silicon-based ceramic substrate, such as a silicon nitride-based or silicon carbide-based material or related materials, either alone or as a composite. Produced by applying an adhesive coating layer.

"Having a compositional gradient" means that a layer material has a compositional gradient with respect to the substrate, from the material below it to which it is deposited, to the material of that layer, and then to the material above the layer of material, if any, to which it is deposited. Just as there are no distinct compositional interfaces between and among layers, coatings (layers) that vary gradually and continuously in composition, although there are no distinct compositional interfaces, microstructural changes, e.g. grain size. changes may or may not occur within the coating.
Lines representing interfaces may or may not be visually discernible. Adheres to substrates even under severe thermal cycling conditions, withstands contact stress, is heat resistant, and resistant to high temperature breakdown or chemical attack2
The deposition of compositionally graded layer coatings over layers depends on careful control of process parameters. The remarkable properties of the coating are the result of the selected coating material and the gradual and continuous change in composition from the substrate to the outer surface of the coating.

The coating according to the invention comprises at least two overlapping layers. Aluminum nitride or aluminum oxynitride is deposited on the substrate as an intermediate adhesion layer and serves as an oxygen diffusion barrier layer. On the intermediate layer, at least one outer layer is deposited, which provides the necessary protection and surface properties for heat engine components, such as oxidation resistance, low oxygen at room and elevated operating temperatures. Provides permeability as well as low coefficient of friction. This coating is applied to a silicon-based ceramic substrate such as reactive bonded silicon nitride (RBSN).
), sintered silicon nitride (SSN), sintered silicon carbide (SS
C), hydrostatic hot pressed silicon nitride (HSN) or the aforementioned U.S. Pat.
667.4.409. It is coated on silicon nitride-based or silicon carbide-based materials, either singly or in composite form, such as the composite silicon nitride materials disclosed in OO 4,4° 421.525 and 4,441.894.

The aluminum nitride or aluminum oxynitride intermediate layer is
The thickness of the first outer layer, preferably an alumina-based composite, is about 0.5 to 20 microns, preferably 2 to 10 microns, while the first outer layer, preferably an alumina-based composite, is about 0.5 to 900 microns, preferably 0.5 to 10 microns thick.
20 microns thick, most preferably 2-10 microns thick.

The coating may include a second outer layer of an alumina-based, zirconia-based or ittria-based composite material.

Additional outer material layers selected to control the surface properties of the coated article can be deposited over the composite layer or layers.

In each aspect of the invention, the outermost layer has a molecular weight of about 0.5 to 900
microns thick and are deposited on layers that are each 0.5 to 20, preferably 2 to 10 microns thick.

The overall thickness of the coating depends on the conditions of the substrate, e.g. roughness,
It depends at least in part on factors such as number of layers, required properties, etc. For thermal barrier coatings, the preferred final thickness of the outermost layer is about 800-900 microns, which can result in an overall coating thickness in excess of 900 microns. Additionally, the final thickness of the outermost layer may also be the result of post-deposition processing, e.g., a layer thicker than the final desired level may be deposited and polished or polished to achieve the final surface properties and/or part dimensions. Machining may then be used.

Thermal expansion mismatch between coating and substrate or between layers can induce high tensile or compressive stresses at the interface between these materials. lists the coefficients of thermal expansion of various materials considered for use as materials, and illustrates the inherent inconsistencies in the materials shown.

Table I * Engineering Property Da
ta on SelectedCeramics,
Vol, I (MCIC Report, 1976
(March) and (MCIC Report, July 1981).

**Range depends on composition.

The gradual change in composition through the thickness of the compositionally graded coating controls the deposition process to produce solid solution regions of sequentially deposited component materials where a gradual change from one material to another occurs. This is realized by These regions of compositional variation or transition regions eliminate well-defined compositional interfaces and represent minimal diffusion conditions between the materials deposited on the substrate. The concentration gradient region or solid solution transition region is sufficiently broad to suppress shear stresses within the coating resulting from thermal expansion induced tensile and/or compressive forces and to produce a coating exhibiting improved adhesion. Such regions range from about 0.1 microns to substantially the entire thickness of the layer.

The method according to the present invention for making the above-mentioned article includes a metal halide and other species reactant gas under carefully controlled conditions to deposit a metal compound by chemical vapor deposition (CVD) onto a substrate. 6 The deposition of compositionally graded coatings according to the present invention involving the use of gas mixtures of different types is achieved by a continuous deposition process, in which the reactant gases introduced into the process are required for a portion of the coating layer. There is a gradual change from the reactants needed in one part to the reactants needed in another part.

The intermediate layer of aluminum nitride and/or aluminum oxynitride and the preferred alumina/zirconia or alumina outer layer are deposited using the following CVD reaction or similar reaction: AIAlCl5(÷NHs(g)= AIN(5)
” 3 HCI(g) (1) 2AIC1s (g) 10CO□ +3H2→A1 Snow 0s(s) + 6 HCI
(g) (2)2AIC1m (g)÷ZrCL (g
)÷5CO* + 5 Hz→ (Al snow Os +
ZrO*)÷10 HCI÷5CO (3) Formula A1.0.
Aluminum oxynitride layers represented by N, (varying stoichiometry) can be deposited by carrying out reactions (1) and (2) simultaneously.

The stoichiometry of the aluminum oxynitride deposited at any given time depends on the reactant to gas ratio introduced into the deposition process along with the AICII.

Reaction (3), under carefully controlled conditions, results in the co-deposition of alumina and zirconia to form a composite two-phase oxide layer in which individual particles of zirconia are distributed in a continuous alumina matrix. Other alumina to zirconia ratios are possible using similar reactions. reaction(
A similar reaction to 3) is (xAt*os÷yYtOs),
(xZrO* + yAlzOs) or (xY*Os
It is also used to deposit a two-phase composite layer of ÷yA1□03).

Deposition of compositionally graded coatings, including composite Altos matrix/Zr0t particles or other species two-phase to more multi-phase composite layers, can be achieved by separating two or more species using reaction (3) or similar reactions. associated with the simultaneous deposition of oxide materials. Such co-deposition also relies on careful control of process parameters, such as the partial pressure of the reactants, to control the relative loadings of the first and additional phase materials. Co-evaporation of these materials is described in detail in Japanese Patent Application Nos. 7655 and 7656, filed by the present applicant.

As described herein, the composite layer includes alumina,
Zero-particle materials in which discrete individual particles of zirconia or yttria or a combination or solid solution thereof are two-phase or more multi-phase layers in which discrete individual particles of zirconia or yttria or a combination or solid solution thereof are distributed in a continuous matrix of alumina, zirconia or yttria or a solid solution thereof. is different from that of the matrix.

An example of a two-phase to more multi-phase layer according to the invention is A
1*Os matrix/ZrO, particles, A1*Os matrix/y*o* particles, A1□0. Matrix/Z
rO* particles + Y2O1 particles, ZrO* matrix/
A 1 ffi 01 particles, and YIO-matrix/A11as particles.

As used herein, second phase, two phases, and two or more phases may refer to the first phase continuous oxide matrix compound, and may be a single compound or two or more compounds in the form of individual grains. It is associated with a composite comprising one or more additional second phases. The particles can be a single metal oxide or a solid solution of one or more metal oxides, and the individual particles can be of the same or different compounds. The particles disclosed herein may have regular shapes such as spheres, rods, whiskers, etc., or may have irregular shapes.

One method of depositing the composite layer is to intermittently pulse the reactant gas needed to form the particulate material or materials into the gas mixture during the deposition of the matrix material, such as alumina. be. Another method is to perform continuous flow co-deposition of Cetrix and particulate materials and adjust the component partial pressures to overcome differences in reactivity.

As can be seen from the above equilibrium reactions and discussion, the composition of the coating deposited at any point during the deposition process depends on the metal halide (if present) and other species reactant gases introduced along with AlCl2 and the relative gas flow rates. Depends on the ratio. Similarly, a compositionally gradient layer of YiOs or ZrO
It can be deposited by gradually decreasing the partial pressure (flow rate) of YC1□ or ZrCL until it is the only metal halide in the gas mixture. Routine testing is used to determine the deposition rate of various coating materials at specific gas flow rates and temperatures. These values will determine the deposition time schedule and flow rate changes necessary to produce the desired coating morphology.

The CVD process described above is the preferred deposition method. However, the coating according to the invention may alternatively be applied by physical vapor deposition (PVD), such as plasma spraying or sputtering.

Examples are presented below, but these are for illustrating the present invention and are not intended to limit the invention.

Degassed in a CVD reactor at 1350 °C,
On a single SSN base material body such as a heat engine part,
A mixture of AlC11 gas in an inert gas was flowed at a substantially constant flow rate. The pressure within the reactor was maintained at approximately 2-5 torr. Ammonia gas was mixed with AlCl, at a flow rate sufficient to begin depositing a continuous layer of aluminum nitride on a silicon nitride substrate. After approx. 4 micron of AIN is deposited,
A portion of the flow of MHI over a period of about 30 minutes is converted from almost completely AIN to almost completely Al*Os with a thickness of about 5 microns by the following equation (2) and reactions (1) and (2) Solid solution of AIN and AhOs (A1.N, O,
) A transition layer is first formed and then about 5 microns thick of pure A.
00 flow was gradually substituted in the flow rate for the AICIg flow rate to deposit the remainder of the 1-Os or nearly pure A1*Os outer layer.

On a degassed sintered composite silicon nitride substrate containing ittria and alumina sintering aids and reinforcing whiskers, an inert carrier gas was applied in a CVD reactor at 975°C and about 5-7 Torr. A gas mixture of AlCl5 and NHs was flowed. The relative flow rates and partial pressures of the gas were first selected to begin depositing a continuous layer of AIN on the substrate surface. After a sufficient time to deposit an approximately 3 micron layer of AIN, the flow of reactant gases is stopped and the flow of reactant gases is stopped to facilitate diffusion of substrate oxygen and silicon from the oxide sintering aid into the AIN layer. The reactor temperature was increased to 135°C while flowing only inert gas.
The temperature was raised to 0°C. After about 30 minutes, the reactant gas flow was resumed, the temperature was lowered to 1050° C., and a portion of the NH, gas flow was gradually replaced with a CO, flow over a period of about 1 hour. This reacts an approximately 5 micron thick aluminum oxynitride composition gradient region that gradually changes stoichiometry from approximately the composition of AIN to approximately the composition of A15Os on the AIN according to (4), (1) and (2). AICII to adhere
This was done by adjusting the flow rate to the flow. Ultimately, the gas stream was made nitrogen-free. Then AlCl5, Cot and H2 in the carrier gas
The flow rate was controlled at a partial pressure ratio (flow rate ratio) selected to initiate deposition of the A1*Om matrix onto the aluminum oxynitride zone by a reaction similar to reaction (3).

ZrCL was pulsed into the open gas mixture for about 5 minutes every 15 minutes at a partial pressure selected to form individual particles of Zr0a in the matrix. Approximately 3 micron thick A
After sufficient time to form a composite layer consisting of a single Os matrix and Zr0m particles, the flow of A,IClm was gradually interrupted and the flow of ZrCL was increased. The flow of other gases in the mixture continued. This goes from an alumina-based composite with progressively increasing % zirconia (the total thickness of the alumina base layer is about 8 microns) to a composite layer about 3 microns thick containing individual alumina particles with progressively decreasing %. approximately 1.5 micron Z on these composite layers.
Sufficient time was allowed to phase the deposited material into an rO layer.

If desired, the adhesion of the coating can be improved by heating during the process (as per Example 2) to increase the interdiffusion of the elements.
Or the vines can be further improved by post-processing. Approximately 1100℃
Above , the compositional gradient of the coating is aided somewhat by diffusion of components between the deposited materials or between the substrate and the deposited material. Diffusion rate is temperature dependent. At temperatures below about 1100° C., the gradient of the coating is substantially dependent on control of the deposition process, particularly in connection with the mixing of reactant gases. Deposition at about 1100-1500° C. has the particular advantage that the substrate components diffuse into the aluminum nitride layer, producing, for example, an aluminum silicon nitride or aluminum silicon oxynitride transition layer underneath the aluminum nitride material. . Alternatively, such a compositionally graded transition layer can be deposited by briefly trapping the desired component (oxidizing gas) into the flow of a reactant stream (eg, AIN-deposited reactant).

As can be seen in Example 2, the deposition temperature need not be constant during the deposition of the compositionally graded coating according to the invention;
9 to control the rate of deposition of the various components of the coating as well as the rate of diffusion during the process as described above.
It can be changed within the range of 00 to 1500°C. The deposition temperature used also depends on the desired microstructure for the substrate and coating and the temperature sensitive components contained in the silicon-based material of the substrate.

Menzu Tachitate 1 The compositionally graded coating according to the present invention has high density and adhesion, and has a clear interface between the layers, and the adhesion and thermal expansion coefficient difference exhibited by overlapping individual successive layers. It is possible to combine the beneficial properties of two or more layers of components without the problems associated with this. The coatings of the present invention are particularly useful for high temperature application components that are subjected to contact stresses, such as turbine rotors in modern gas turbine engines.

Although preferred embodiments of the invention have been described, it should be understood that many changes may be made within the spirit of the invention.

Claims (1)

[Scope of Claims] 1) A high-density molded substrate body made of a silicon nitride-based or silicon carbide-based single or composite material, and an aluminum nitride or silicon carbide-based material chemically bonded onto the substrate body. a coherent, continuous, dense interlayer of about 0.5 to 20 microns thick, comprising an aluminum oxynitride material, and an oxide of aluminum or zirconium chemically bonded onto the interlayer; a coherent, continuous, dense first outer layer between 0.5 and 900 microns thick, where each layer is further deposited from the underlying material to which it is deposited to the material of that layer. A coated article that is a compositionally graded layer that varies in composition, up to the material above the material layer, if any, such that there are no sharp compositional interfaces between the substrate and the layer and between the layers. 2) The article of claim 1, wherein the intermediate layer is between 2 and 10 microns thick. 3) The article of claim 1, wherein the first outer layer is 0.5 to 20 microns thick. 4) The article of claim 3, wherein the first outer layer is 2 to 10 microns thick. 5) the first outer layer consists of a composite ceramic material having a compositional gradient consisting of a continuous layer of alumina or zirconia in which discontinuous individual particles are dispersed; The article according to claim 1, which is at least one material selected from the group consisting of: and yttria. 6) The first outer layer is 0.5-20 microns thick and about 0.5-90 microns chemically bonded onto the first outer layer.
further comprising a coherent, continuous, dense second outer layer of zero micron thickness, the second outer layer comprising a composite ceramic material having a compositional gradient different from the material of the first outer layer; the outer layer consists of a continuous layer of alumina, zirconia or yttria in which discrete individual particles are dispersed, the particles being different from the continuous layer and of at least one material selected from the group consisting of alumina, zirconia and yttria; The article according to claim 5, which is 7) The second outer layer is 0.5 to 20 microns thick and is comprised of alumina, zirconia, yttria stabilized zirconia, or yttria and is chemically bonded onto the second outer layer. 7. The article of claim 6 further comprising a cohesive, continuous, dense additional outer layer 900 microns thick. 8) The first outer layer is 0.5 to 20 microns thick and is comprised of alumina, zirconia, yttria stabilized zirconia, or yttria and is chemically bonded onto the first outer layer. The article of claim 1 further comprising a coherent, continuous, dense additional outer layer 900 microns thick. 9) A dense molded substrate body of silicon nitride-based or silicon carbide-based single or composite material and an aluminum nitride material approximately 2 to 10 microns thick chemically bonded onto the substrate body. a cohesive, continuous dense interlayer comprising: a continuous layer of alumina having discontinuous individual zirconia particles dispersed therein chemically bonded onto said interlayer;
A close-fitting, continuous high-density first layer approximately 2-10 microns thick.
A cohesive, continuous, high-density, approximately 2-10 micron thick, consisting of an outer layer and a continuous layer of zirconia chemically bonded and dispersed with discrete individual alumina particles on the first outer layer. a second outer layer and a cohesive, continuous, dense additional outer layer of zirconia, approximately 1-5 microns thick and chemically bonded onto the second outer layer, where each layer comprises: There should be no distinct compositional interface between the substrate and the layer and between the layers, from the underlying material to which it is deposited, to the material of the layer in question, and then to the upper material, if any, of the material layer to which it is deposited. and a coated article that is a compositionally graded layer that varies in composition. 10) A method of depositing a high temperature stress and oxidation resistant coating on a silicon nitride-based or silicon carbide-based substrate body, the method comprising:
a gaseous mixture of one or more halide vapors, other suitable reactant gases, and optionally a carrier gas at a pressure in the approximately ambient pressure range sufficient to deposit a continuous, dense, adherent coating on the substrate. flowing onto the substrate at a partial pressure and flow rate of and for a sufficient period of time, during deposition of the coating, the coating has a compositional gradient including at least two adhesion layers;
Each layer has no distinct compositional interfaces between the substrate and the layer and between the layers, from the underlying material to which it is deposited to the material of that layer and then to the upper material if there is a layer of material deposited thereon. grading the halide vapor and other suitable reactant gases in such a manner that the vapors and gases and their The partial pressure is varied according to a predetermined time schedule, and the halide vapor and other suitable reactant gases are applied to a substrate approximately 0.5 to 20 microns thick, in which at least two layers are chemically bonded onto the substrate body. a cohesive, continuous, dense interlayer of aluminum nitride or aluminum oxynitride material, and an oxide of aluminum or zirconium chemically bonded onto the interlayer, and having an oxide of about 0.5 to 90%;
a coherent, continuous, dense first outer layer of 0 micron thickness; 11) the step of grading the vapors and gases comprises a composite ceramic material having a compositional gradient in which the first outer layer consists of a continuous layer of alumina or zirconia dispersed with discrete individual particles; 10. The method of claim 10 comprising selecting the predetermined time schedule and the vapors and gases and their partial pressures such that the layer is different from a continuous layer and is selected from the group consisting of alumina, zirconia and yttria. Method. 12) such that the first outer layer is 0.5 to 20 microns thick and at least two layers are comprised of a composite ceramic material having a compositional gradient different from the first outer layer material and the first outer layer is 0.5-20 microns thick; about 0.5 chemically bonded onto the layer
further comprising a coherent, continuous, dense second outer layer ~900 microns thick, the second outer layer consisting of a continuous layer of alumina, zirconia or zirconia having discontinuous individual particles dispersed therein; The step of grading the vapors and gases is different from a continuous layer and is of at least one material selected from the group consisting of alumina, zirconia and yttria. 11. The method of claim 10, comprising selecting a pressure. 13) Gradually changing the vapor and gas such that the second outer layer is 0.5-20 microns thick and at least two layers are comprised of alumina, zirconia, yttria-stabilized zirconia or yttria; a predetermined time schedule and vapors and gases and their 13. The method of claim 12, comprising selecting a partial pressure. 14) Gradually changing the vapor and gas such that the first outer layer is 0.5-20 microns thick and at least two layers are comprised of alumina, zirconia, yttria stabilized zirconia or yttria; 1
a predetermined time schedule and vapors and gases and their partial pressures to further include a coherent, continuous, dense additional outer layer, about 0.5 to 900 microns thick, chemically bonded onto the outer layer; 11. The method of claim 10, comprising selecting.